Alkylation is a reaction in which an alkyl group is added to an organic molecule. Thus, an isoparaffin can be reacted with an olefin to provide an isoparaffin of higher molecular weight. Industrially, the concept depends on the reaction of a C.sub.2 to C.sub.5 olefin with isobutane in the presence of an acidic catalyst producing a so called alkylate. This alkylate is a valuable blending component in the manufacture of gasolines due not only to its high octane rating but also to its ability to meet the compositional requirements of reformulated gasolines.
Industrial alkylation processes have historically used hydrofluoric or sulfuric acid catalysts under relatively low temperature conditions. The sulfuric acid alkylation reaction is particularly sensitive to temperature, with low temperatures being favored to minimize the side reaction of olefin polymerization. Acid strength in these liquid acid catalyzed alkylation processes is preferably maintained at 88 to 94 weight percent by the continuous addition of fresh acid and the continuous withdrawal of spent acid. The hydrofluoric acid process is less temperature sensitive than the sulfuric process and is more easily recovered and purified.
Both hydrofluoric and sulfuric catalysts are gradually depleted in continuous alkylation processes and must be regenerated or replenished by mixture with fresh acid to maintain acid strength, reaction rate, and the resulting alkylate quality. Specifically, alkylate quality responds directly to increasing acid strength, and the acid makeup or regeneration rate is typically controlled together with other process variables such as temperature and space velocity, to meet a required alkylate quality specification. Both sulfuric acid and hydrofluoric acid alkylation share inherent drawbacks including environmental and safety concerns, acid consumption and sludge disposal. For a general discussion of sulfuric acid alkylation, see the series of three articles by L. F. Albright et al., "Alkylation of Isobutane with C.sub.4 Olefins", 27 Ind. Eng. Chem. Res., 381-397 (1988). For a survey of hydrofluoric acid catalyzed alkylation, see 1 Handbook of Petroleum Refining Processes 23-28 (R. A. Meyers, Ed. 1986). Catalyst complexes comprising BF.sub.3 as well as BF.sub.3 : H.sub.3 PO.sub.4 adducts have been proposed by co-pending and co-assigned U.S. applications Ser. Nos. 608,783, 790,324 and 608,856. These applications are herein incorporated by reference. In co-pending application Ser. No. 608,783 a catalyst complex comprising the reaction product of BF.sub.3 and H.sub.3 PO.sub.4 was used in molar ratios of 0.5:1 to about 1.5:1 with the addition of excess BF.sub.3 in concentrations of about 10 ppm to about 5% by weight of the total feed stock. In co-pending application Ser. No. 608,763, a catalyst complex comprising BF.sub.3 : H.sub.3 PO.sub.4 adduct and at least one polar hydrocarbon formed in situ was used to avoid alkylation byproducts and improve isoparaffin: olefin alkylation selectivity. In co-pending application Ser. No. 790,324 alkylation is accomplished using a catalyst complex comprising a Lewis acid and at least one material selected from the group consisting of a protic solvent having a pKa less than about 16, a zeolitic or nonzeolitic solid and an ion exchange resin. The reaction takes place under alkylation conditions including temperatures from about -40.degree. to about 500.degree. C., and pressure from subatmospheric to about 5000 psig.
U.S. Pat. No. 2,345,095 to Bruner teaches a paraffin-olefin alkylation process catalyzed by a boron tri-fluoride-water complex, represented by the formula BF.sub.3 :nH.sub.2 O, where n is preferably from 1 to 1.5. The Brunet reference notes on page 2 left hand column, lines 13-23, that the BF.sub.3 :H.sub.2 O catalyst complex behaves similarly to sulfuric acid but is a superior alkylation catalyst because BF.sub.3 :H.sub.2 O does not promote oxidation to undesired byproducts.
U.S. Pat. Nos. 2,296,370 and 2,296,371 to Slotterbeck disclose a BF.sub.3 :H.sub.2 O:HF catalyst system and an isoparaffin-olefin alkylation process employing the same. The catalyst system is said to avoid yield loss due to oxidation of the resulting alkylate product. The Slotterbeck '370 and '371 patents also discuss loss of catalytic activity due to diminishing acid strength; see the Slotterbeck '370 patent at page 2, right hand column at line 75 through page 3, left hand column at line 55 and the Slotterbeck '371 patent at page 2, right hand column at line 66, through page 3, left hand column at line 41. U.S. Pat. No. 3,873,634 to Hoffman teaches a method of increasing the rate of ethylene alkylation by isobutane by carrying out the reaction simultaneously with the alkylation of a small amount of a higher weight olefin in the presence of a BF.sub.3 :H.sub.3 PO.sub.4 catalyst complex at low temperature and pressure.
U.S. Pat. No. 3,925,500 to Wentzheimer discloses a combined acid alkylation and thermal cracking process employing a BF.sub.3 :H.sub.3 PO.sub.4 acid catalyst in which unconverted propane and ethane from the alkylation process are converted, for example, to propylene and ethylene which are subsequently alkylated with isobutane to yield a liquid fuel.
The use of downflow reactors in petroleum processing has generally been for catalytic conversions using solid bed catalysts. For example, U.S. Pat. No. 4,797,262 to Dewitz discloses an integral hydrocarbon conversion catalytic cracking conversion apparatus for the catalytic conversion of hydrocarbons comprising a catalytic downflow reactor, an upflow catalytic riser regenerator and a cyclonic separator for separating out spent catalysts. The separator interconnects the exit of the downflow riser reactor with the inlet of the upflow riser regenerator by means of a pressure leg seal. Solid catalysts disclosed are aluminosilicates and metal oxides such as magnesium or zirconium. U.S. Pat. No. 3,976,713 to Holmes discloses an isoparaffin-olefin alkylation process using granular catalyst solids in a plurality of beds packed in series. The effluent of each bed is recycled to the inlet of said bed. The feed stream is fed downward through conventional distribution means through the solid bed catalysts. U.S. Pat. No. 2,363,222 to Beyerstadt relates to the use of boron trifluoride in phosphorous acids as a catalyst for the alkylation of isoparaffins with mono-olefins. This reference disclosed the preparation of a catalyst composition by bubbling boron trifluoride through the phosphorous acid and saturating the acid. This reference discloses that it is essential that the feed stock contain at least one paraffinic hydrocarbon containing at least one tertiary carbon atom per molecules and at least one olefin. The process may be carried out either as a batch or continuous process.
U.S. Pat. No. 4,385,985 discloses the use of a downflow riser in a fluid catalytic cracking process. The downflow riser is disclosed as affecting uniform distribution of the catalyst throughout the feed, decreasing the contact time of the catalyst with the feed and decreasing the amount of coke made in the process. The riser of the reaction vessel is placed on top of the reactor in such a manner that it forces the downflow movement of the regenerated catalyst mixed with the petroleum feedstock. Those catalyst disclosed are zeolites, silica-alumina and carbon monoxide burning promoters such as platinum metals e.g., platinum, palladium, rhodium, ruthenium, iridium and osmium. This process uses an apparatus which includes a riser mounted on a reaction vessel, a steam stripper section and a catalyst regenerator riser.
U.S. Pat. No. 2,401,884 to Schulz et al. discloses an alkylation process using liquid catalysts such as boron tri-fluoride which are pretreated with an olefin having a fewer number of carbon atoms per molecule than the olefin used as the principle alkylating reactant. An inorganic catalyst complex with boron tri-fluoride is prepared and pretreated with, for example, ethylene prior to contact with mixtures containing isoparaffins and higher olefins. This complex is said to be more stabilized and resistent to degradation than the catalysts would be without pretreatment.
Catalysts complexes comprising BF.sub.3 as well as BF.sub.3 :H.sub.3 PO.sub.4 adducts have overcome many of the safety and environmental draw backs of sulfuric and hydrofluoric acid alkylation systems. However, the volume and quality of alkylates using these catalysts has not always been comparable to that of sulfuric or hydrofluoric acid alkylates.
Traditional catalytic isoparaffin-olefin alkylation processes typically require excess isoparaffin and generally exhibit a direct relationship between increasing isoparaffin concentration and alkylate octane quality. The isoparaffin is an expensive feedstock and for this reason the isoparaffin is typically separated from the alkylate product stream and recycled to the alkylation reaction zone. The isoparaffin:olefin ratio for alkylation in the presence of certain BF.sub.3 -containing catalysts complexes must typically exceed about 5:1 to produce an alkylate of acceptable quality.
BF.sub.3 :H.sub.3 PO.sub.4 catalyst complexes have a relatively high viscosity, making the use of standard upflow reactors typically used for HF alkylation somewhat impractical. Up until now, however, only upflow reactors have been used with these catalyst systems. This is because the continuous phase of standard alkylation processes is generally the acid phase. Due to the high viscosity of the acid phase in phosphoric promoted alkylation processes, a great deal of energy is required in stir reactors as well as other types of reactors which use upflow systems for generating alkylated product. The instant invention takes advantage of the inherent high viscosity and high density differences between the phosphoric acid phase and the hydrocarbon. As opposed to using the conventional continuous acid phase methods whereby hydrocarbon is dispersed in the acid medium to insure excess catalyst, the present invention uses the hydrocarbon as the continuous phase whereby the catalyst acid complex is disbursed into the top of the downflow reaction vessel. Thus, the invention provides a uniform dispersion of catalyst acid complex in a hydrocarbon continuous phase without the use of energy intensive stir reactors or the like. The result is a lower cost process with desirable octane yield and catalyst activity.